1. Field of the Invention
The present invention relates to a touch sensor and, more particularly, to an electrical touch sensor for electrically detecting whether an object is touched and informing the result and a human interface device using the same.
2. Description of Related Art
Generally, a touch sensor is classified into a push button operated in a mechanical manner, and an electrical touch sensor operated in a non-mechanical manner.
The push button requires a mechanical contact for detecting whether an object pushes a button, and a spring device for turning the button back, to make it difficult to minimize the resultant product and make its structure complicated, while its manufacturing cost is inexpensive. On the other hand, the electrical touch sensor is advantageous to minimize the product, while its manufacturing cost is more expensive than the push button.
The present invention is especially directed to the electrical touch sensor.
FIGS. 1A to 1C are views illustrating a circuit diagram of a conventional electrical touch sensor.
FIG. 1A is a basic circuit diagram of the electrical touch sensor, FIG. 1B is an operating circuit diagram of the electrical touch sensor when a contact object is not in contact with the sensor, and FIG. 1C is an operating circuit diagram when the contact object is in contact with the sensor.
Referring to FIG. 1A, the electrical touch sensor includes two touch pads (or touch pins) PAD1 and PAD2 for touching an object, a first resistor R1 for protecting an inner circuit from static electricity transmitted from the object, an active device N-type FET Q1, a second resistor R2 for determining a bias voltage of the N-type FET Q1, a third resistor RL for acting as a load of the N-type FET Q1, and an output buffer for buffering an output voltage of the N-type FET Q1.
When the object is not in contact with the electrical touch sensor, as shown in FIG. 1B, the touch pads PAD1 and PAD2 are opened, and therefore, a gate of the N-type FET Q1 is connected to a ground voltage through the second resistor R2.
The N-type FET Q1, through which the ground voltage and the gate are connected, turns off, and no current flows through the N-type FET Q1 and the third resistor RL. Therefore, a power source voltage VDD is applied to a drain of the N-type FET Q1, and the output buffer B1 receives the power source voltage VDD applied to the drain of the N-type FET Q1 to output a high level of signals.
On the other hand, when the object is in contact with the electrical touch sensor, the electrical touch sensor has the operating circuit diagram of FIG. 1C.
At this time, generally, the object may be a human's finger which is resistance having conductivity.
Referring to FIG. 1C, the two touch pads PAD1 and PAD2 are connected through a resistance RH, through which current generated by voltage difference of the power source voltage VDD and the ground voltage VGND flows. Hereinafter, the resistance RH of the human in contact with the touch pads is referred to as a fourth resistor RH.
As a result, a voltage of “VDD×R2/(R1+R2+RH)” is applied to the gate of the N-type FET Q1.
In addition, when the voltage applied to the gate of the N-type FET Q1 is larger than a threshold voltage Vth of the N-type FET Q1, the N-type FET Q1 turns on so that drain current IL of “A×(VG−Vth)2” flows through the N-type FET Q1 and the third resistor RL. Here, A is a specific constant of a FET, VG is a gate voltage of a FET, and Vth is a threshold voltage of a FET, That is, a voltage VD of “VDD−IL×RL” is applied to the drain of the N-type FET Q1.
Especially, when a voltage to flow the drain current IL calculated as “IL×RL>VDD” is applied to the gate of the N-type FET Q1, the drain voltage of the N-type FET Q1 becomes “0”.
As described above, when an object having a very small resistor value is in contact with the two touch pads PAD1 and PAD2, a drain voltage having an approximately “0” value is generated in the drain of the N-type FET Q1 to be outputted as a low level of signals through the output buffer B1.
The conventional electrical touch sensor includes the two touch pads to electrically detect whether the object having conductivity is in contact with both of the two touch pads, thereby outputting signals corresponding to the detected result.
While the conventional electrical touch sensor is applicable to various electric/electronic devices by converting only an electrical contact to an electrical signal, without mechanical components, the two touch pads should be required in order to detect whether the object is in contact with the sensor.
Therefore, it is difficult to minimize the conventional electrical touch sensor since it should have the two touch pads.
In addition, since the conventional electrical touch sensor detects whether the object is in contacted with the touch pads using a resistor generated from a human body and so on, the object should have a certain resistance.
When the object has insufficient conductivity, i.e., when a user wears gloves having no conductivity, or when the user's hand is dry, though the user's finger touches the electrical pads, the electrical touch sensor generates output signals as shown in FIG. 2.
That is, when the object has insufficient conductivity, though the object is in contact with the touch pads PAD1 and PAD2 (sections 1 and 3), the touch pads PAD1 and PAD 2 are in the same state that a very high resistor is connected to the touch pads PAD1 and PAD2. As a result, a minor current is applied to gate terminal of the N-type FET Q1 to make the electrical touch sensor generate a high level of signals.
As described above, in the case of the conventional electrical touch sensor, when the object has insufficient conductivity though the object is in contact with the pads, the electrical touch sensor may not detect the contact to be malfunctioned.